When conducting a hydraulic fracturing operation, a hydraulic fracturing fluid is pumped into a subterranean formation under sufficient pressure to create, expand, and/or extend fractures in the formation and to thus provide enhanced recovery of the formation fluid. Hydraulic fracturing fluids typically comprise water and sand, or other proppant materials, and also commonly include various types of chemical additives. Examples of such additives include: gelling agents which assist in suspending the proppant material; crosslinkers which help to maintain fluid viscosity at increased temperatures; gel breakers which operate to break the gel suspension after the fracture is formed and the proppant is in place; friction reducers; clay inhibitors; corrosion inhibitors; scale inhibitors; acids; surfactants; antimicrobial agents; and others.
Fracturing operations have long been conducted in both low permeability and high permeability formations in order, for example, to increase the rate of production of hydrocarbon products or to increase the injection rates of water or gas injection wells. Moreover, with the introduction of slickwater fracturing procedures which use large quantities of water containing friction reducers, it is now also possible to stimulate naturally fractured shales by fracturing multiple intervals during staged treatments in horizontal wellbores. Treatment of all zones of interest in a horizontal well may require several hours to a few days to complete.
Heretofore, when conducting hydraulic fracturing operations in vertical wells, well logging, microseismic, or other techniques have be used to determine production rates and/or the position, length, and height of each fracture. However, when, for example, a horizontal well extending through a shale formation is fractured in multiple stages, microseismic analysis is essentially unable to determine which of the fractured stages are successfully producing oil and/or gas products and which are not. Moreover, the impeller apparatuses used in production logging tools do not function satisfactorily in horizontal wells. Therefore, neither technique is able to reliably determine (a) whether production is occurring from any given stage, (b) the amount of production from any given stage, or (c) the comparative amounts of production from multiple stages.
Consequently, a need has long existed for a method for reliably (a) confirming that crude oil or other liquid hydrocarbon products are being produced from specific fractured zones, (b) determining the rate of liquid hydrocarbon production from a fractured formation zone, or (c) determining the comparative rates of liquid hydrocarbon production from multiple fractured zones, particularly in horizontal wells. Such information would be of great benefit to the operator in (1) identifying possible actions or repairs which would provide immediate improvement, (2) selecting and optimizing enhanced recovery procedures, (3) optimizing the operation of an enhanced lifting system used in the well, (4) reducing water production and the lifting costs associated therewith, and (5) optimizing the performance and cost effectiveness of fracturing and other completion procedures used in other wells drilled in the same field.
Water soluble chemical tracers have been used heretofore in hydraulic fracturing operations to trace the return of the aqueous fracturing fluid. These water soluble tracers are intended to dissolve in and flow with the aqueous fracturing fluid. Thus, they are only able to provide an indication of (a) how much of the fracturing fluid is recovered from, or undesirably remains in, the formation and (b) the comparative recovery of the fracturing fluid, or lack thereof, from one fractured zone versus another.
Consequently, water soluble tracers used for tracing the return or loss of the injected fracturing fluids are not capable of determining whether any hydrocarbon product is actually being produced from a particular zone of a multi-zone well or how much hydrocarbon product is being produced from one zone versus another.
In addition, attempts made heretofore by those in the art to develop and use oil soluble tracers to trace oil production from fractured zones have not been satisfactory. One approach attempted heretofore has been to deliver viscous tracer emulsions into fractured zones. However, such emulsions can be broken, for example, by (1) the heat within the formation, (2) the pumping and formation pressures to which the emulsions are subjected, (3) the shear forces exerted on the emulsions during pumping and injection, and (4) exposure to water flow within the subterranean formation. In addition, such emulsions commonly have a low specific gravity such that the emulsions can separate and accumulate in higher regions of the fractured zone. Consequently, when attempting to evaluate the tracer analysis, the operator cannot be confident that a significant amount of the tracer emulsion (a) was not pushed or washed out of the fractured zone, (b) did not drift and accumulate in higher pockets, or (c) was even properly received in the fractured zone in the first place.